Generally, light of wavelength in the range of 10 nanometers (nm) to 400 (nm) is considered ultraviolet and several sub-ranges of ultraviolet light are often referred to including UVA (generally, 315 nm-400 nm) and UVB (generally, 280 nm-315 nm), for example. The spectrum of solar irradiance, while also including light in the visible and infrared ranges, includes light in the ultraviolet range and specifically in the UVA and UVB ranges. While the vast majority of ultraviolet radiation emitted by the sun never reaches the Earth's surface due to the Earth's atmosphere, the small percentage of light in the ultraviolet range that does reach the Earth's surface has a significant impact on human life.
Exposure to UV radiation from the sun can have both harmful and beneficial effects on the human body. One of the more visible effects on the human body of exposure to UV light is a change in skin pigmentation and resulting color. Melanin, a brown pigment in the skin, increases in concentration when exposed to UV light in order to absorb UV radiation and dissipate the associated energy. Melanin production is a defensive mechanism which, depending on exposure level, can make the skin more resistant to sunburns. However, insufficient melanin production can result in DNA damage leading to skin cancers such as melanoma, as is commonly known. It has also been shown that melanin production does not decrease the harmful effects of UVA which is most commonly attributed to skin aging and damage leading to skin cancers.
Mitigating techniques such as sunscreens, sun blocks, creams, lotions, and/sprays have been developed and are increasingly effective at filtering UV light before it contacts skin cells by absorbing and/or reflecting radiation in both the UVA and UVB ranges. Sunscreens generally have a sun protection factor (SPF) indicating the level of protection against UV, and particularly UVB, light. For example, application of a sunscreen having an SPF of 15 will only cause the skin to exhibit erythema, or a reddening or burning of the skin, when exposed to 15 times the radiation that would otherwise cause the skin to burn. While UVA radiation is less likely to cause erythema, it has been found to cause DNA damage at a deeper level within the skin. Accordingly, several sunscreens containing zinc oxide and/or titanium dioxide, for example, have been developed to effectively block UVA radiation. In order to provide a relative scale, the persistent pigment darkening (PPD) and protection factor A (PFA) methods have been developed, such as that taught, for example, by U.S. Pat. No. 7,033,577, incorporated herein by reference, to indicate levels of protection against UVA radiation.
While too much exposure to UV light can be harmful, too little exposure can also have detrimental effects. For example, UV light, and particularly light in the UVB range, induces the production of vitamin D in the skin. Vitamin D has immunological and cardiovascular effects, among others, and therefore increased health of the human body requires that its concentration and production be appropriately regulated.
While suggested daily exposures of UV radiation can be retrieved from sources such as the World Health Organization and the U.S. Environmental Protection Agency, generally in the form of a UV index, several personal, environmental, and geographical factors, among others, effect any determination of a person's recommended daily exposure. Accordingly, there is a need for a system to assist a user in effectively managing UV exposure, daily and/or over an extended period of time in order to receive a more balanced and healthy amount of both UVA and UVB radiation.